What causes previously stable continental crust in the forelands of Cordilleran orogenic systems to shorten during low-angle subduction? The National Science Foundation/EarthScope Bighorn Project combined seismic imaging of the crust and Moho with kinematic modeling of Laramide (Late Cretaceous–Paleogene) basement-involved deformation to address this question. In north-central Wyoming, asymmetrical ENE-verging upper-crustal folds are highly discordant with broader, N-trending warps in the Moho, indicating crustal detachment. Restorable cross sections of ENE-directed detachment at a depth of ~30 km, combined a smaller component of NNW–SSE shortening due to the east-narrowing shape of the crustal allochthon, can explain the anastomosing network of Laramide basement-cored arches without major deformation of the underlying mantle lithosphere.
Thrust-related fold geometries and west-to-east initiation of deformation in the Laramide and Sevier thrust belts point to Cordilleran end-loading from the west. Differences between Laramide (~N65E) and plate (~N25E) convergence directions, along with the fanning of Laramide shortening directions from nearly E–W to the south to NE–SW to the north, indicate slip partitioning during end-loading west of the Rockies.
Sub-horizontal detachment with a near-zero critical taper within cratonic crust suggests an extremely weak Laramide detachment zone during deformation. Analogous lower-crustal deformation in subduction forearcs is associated with slow earthquakes and slab dehydration. We hypothesize that low-angle subduction of the Farallon Plate suppressed fluid-consuming melting and corner-flow processes that characterize higher-angle subduction. This allowed subduction-generated fluids to escape upward into the overlying continental lithosphere, causing retrograde metamorphism and increased fluid pressure that facilitated crustal detachment. This hydration-based hypothesis predicts that crustal detachment will accompany major earthquakes in active analog orogens.
Convergent forelands in Cordilleran orogenic systems, where continental crust deforms during the subduction of oceanic crust, are difficult to link to plate processes, because they can be more than 1,000 km from the closest subduction trench and more than 80 km above the down-going slab (Barazangi and Isacks, 1976; Gutscher et al., 2000; Martinod et al., 2010). Some forelands remain largely undeformed, like the craton east of the Canadian Rocky Mountains, whereas others are pervasively shortened, like the Laramide orogen in the U.S. Rocky Mountains (Fig. 1). These diverse responses to plate convergence in Cordilleran orogenic systems can provide insights into the larger controls on continental deformation.
This paper investigates two questions relating to earth tectonics: (1) what drives the unusually far-field foreland deformation commonly associated with low-angle subduction; and (2) what processes cause the deformation of the previously stable continental crust in these forelands? This paper focuses on the Laramide orogen (Fig. 1) in the U.S. Rocky Mountains (Late Cretaceous–Paleogene; Dickinson et al., 1988; Copeland et al., 2017), where a previously stable craton was shortened in the foreland of a Cordilleran orogenic system undergoing low-angle subduction. We summarize results from the National Science Foundation (NSF)/EarthScope Bighorn Project, which collected 3D seismic (Bighorn Arch Seismic Experiment (BASE): Yeck et al., 2014; Worthington et al., 2016) and structural (Aydinian, 2020) data from north-central Wyoming and combine these with recent Cordilleran models (e.g., Hyndman et al., 2005; Chapin et al., 2014), previous geophysical imaging (e.g., Smithson et al., 1979; Liu et al., 2010; Jones et al., 2015), and analyses of Laramide kinematics (e.g., Erslev and Koenig, 2009; Yonkee and Weil, 2015).
We conclude that lithospheric end-loading west of the Laramide foreland during Cordilleran convergence drove crustal detachment in the previously stable Rocky Mountain lithosphere. We hypothesize that fluids released during low-angle subduction escaped upward into the foreland crust, catalyzing basement-involved shortening by regional crustal detachment.
CORDILLERAN OROGENS AND THEIR FORELAND THRUST SYSTEMS
The forelands of Cordilleran orogenic systems, where oceanic plates subduct beneath continental lithosphere, are geographically separated from their subduction trenches by complexly faulted hinterland belts that commonly contain Andean-style arc complexes. The mountainous elevations and upper-crustal thrust belts of many shortened Cordilleran systems have led to the assumption that the lower crust must thicken at depth to match the upper-crustal shortening. But seismic transects through several Cordilleran hinterlands (Clowes et al., 1995, 2005; Fuis et al., 2008; Cook et al., 2010) have imaged relatively thin crust (Hasterok and Chapman, 2007). For example, crustal thicknesses in the Canadian Cordillera’s backarc hinterland (30–35 km thick) are consistently thinner than crustal thicknesses in the craton to the east (40–45 km thick; Kao et al., 2014). In response to these observations, Hyndman et al. (2005, 2009) and Hyndman and Currie (2011) explained many high Cordilleran elevations with thermal density reductions (Perry et al., 2002; Hyndman and Currie, 2011). This hypothesis is supported by seismic velocity (Hyndman et al., 2009; Kaban et al., 2014) and xenolith (Canil, 2008) data indicating higher Moho temperatures in hinterlands (800–900°C) than in their adjoining cratons (400–500°C).
Because thrust-shortened Cordilleran hinterlands are typically underlain by a relatively flat Moho with less structural relief and smaller fault offsets than in their overlying thrust belts (Cook, 1995), it appears that continental crust may have (1) been converted into arc volcanic rocks (DeCelles et al., 2009, 2015); (2) undergone subduction erosion/delamination (Meissner and Mooney, 1998; Currie et al., 2008); and/or (3) been modified by subsequent lower crustal flow. Asthenospheric corner flow, driven by the subducting plate, may provide both a source of backarc heat and a mechanism for removing continental mantle and lower crust from Cordilleran hinterlands (Hyndman et al., 2005; Chapman et al., 2017).
The co-location of the abrupt eastern limit of the Canadian foreland thrust belt (Dahlstrom, 1970; Price, 1981) with a sharp contrast in Moho temperatures led Hyndman et al. (2009) to propose that the thermal state of the lower crust determined whether the craton deformed or remained stable. But not all Cordilleran systems, especially those formed during low-angle subduction, have sharp transitions between large-displacement thrust faults and largely unshortened continental crust. For example, the Laramide Rocky Mountain orogen of the western U.S. (Fig. 1) is a wide zone of basement-involved thrust faulting sandwiched between the higher-slip, predominantly thin-skinned Sevier thrust belt to the west and the stable North American craton to the east. During the Laramide orogeny, the role of lower-crustal temperatures in limiting Cordilleran deformation is unclear, because Moho temperatures appear to have been more craton-like than backarclike (Currie and Beaumont, 2011; Tesauro et al., 2014).
Cordilleran Basement-Involved Foreland Thrust Belts
Over the past 50 years, geometric observations (Gries, 1983; Schmidt and Perry, 1988; Erslev, 1993; Schmidt et al., 1993; Stone, 1993; Ramos et al., 2002, 2004; Pearson et al., 2013; Lacombe and Bellahsen, 2016), kinematic data (Blackstone, 1980; Erslev and Koenig, 2009; Yonkee and Weil, 2015), and geophysical imaging (Smithson et al., 1979; Stone, 1985a, 1985b; Cahill and Isacks, 1992; Gutscher et al., 2000; Devlin et al., 2012) have shown that many Cordilleran basement-involved forelands are thrust belts dominated by horizontal shortening. Focal plane mechanisms in active analog orogens (e.g., the Sierras Pampeanas of Argentina: Barazangi and Isacks, 1976; Jordan and Almendinger, 1986; Alvarado et al., 2009; Richardson et al., 2011; Devlin et al., 2012) confirm the primary role of thrust and reverse faulting in these orogens.
In Cordilleran forelands, basement-involved “thick-skinned” and more classical “thin-skinned” thrust belts (Dahlstrom, 1970; Price, 1981) differ in important ways. Thin-skinned thrust belts imbricate layered sedimentary rocks at the surface, forming regular arrays of thrusts and folds in sedimentary prisms shortened into critical-taper wedges (Davis et al., 1983; McClay, 2004). In contrast, thick-skinned thrust belts typically shorten both thin platformal sedimentary rocks and their underlying basement rocks, forming anastomosing networks of basement-cored arches separated by elliptical sedimentary basins. Relative to thin-skinned thrust belts, thick-skinned thrust belts typically show less slip on more widely spaced and diversely oriented faults.
Thin-skinned thrust belts are mostly univergent, with most major thrusts slipping away from the orogenic welt toward the continental interior. In contrast, basement-involved foreland thrust belts are more bivergent, with major thrusts slipping both toward and away from the orogenic welt. For example, most major thrusts in the thin-skinned Wyoming-Utah-Idaho Sevier thrust belt slipped toward the continent, whereas master thrusts in the adjacent Laramide orogen (Fig. 1) slipped both toward (e.g., Beartooth and Bighorn arches) and away (e.g., Wind River and Owl Creek arches) from the continental interior. Some Laramide basement-cored arches have semi-equivalent ENE- and WSW-directed thrust slip on both arch flanks (e.g., Uinta and Front Range arches).
Some differences between thin- and thick-skinned Cordilleran thrust belts appear to be due to differing amounts of shortening, not due to fundamentally different processes. In Wyoming, Weil and Yonkee (2012) calculated similar layer-parallel shortening (LPS) directions from both macroscopic cleavage in the thin-skinned Sevier thrust belt and more cryptic cleavage revealed by anisotropy of magnetic susceptibility (AMS) measurements in the thick-skinned Laramide thrust belt. Because these LPS directions parallel the shortening directions of adjacent minor faults, their tectonic origin is clear (Yonkee and Weil, 2015).
Rheological contrasts between thin- and thick-skinned Cordilleran thrust belts can explain many of their differences. The layered sedimentary stratigraphy in thin-skinned thrust belts facilitate flexural slip folding in kink-band ramp-flat thrust-fold geometries (Suppe, 1983). More irregular pre-existing weaknesses in the crystalline basement of thick-skinned thrust belts result in rotational fault-bend folding (Erslev, 1986), with many basement-involved structures following earlier faults, shear zones, and basement foliations (Stone, 1986; Brown, 1993; Marshak et al., 2000; Sims et al., 2001; Timmons et al., 2001; Karlstrom et al., 2005; Magnani et al., 2005; Chapin et al., 2014; Worthington et al., 2016). These differences in pre-existing weakness orientations can explain the greater diversity of fault strikes and fold trends in thick-skinned belts relative to those in thin-skinned belts.
The fluid regimes of thin- and thick-skinned belts can also be expected to differ. Sedimentary strata in thin-skinned thrust belts typically produce synkinematic fluids by compaction and diagenetic reactions, generating high fluid pressures and mineralogical changes that can facilitate deformation. In contrast, basement rocks in thick-skinned thrust belts commonly equilibrated at higher temperatures than those associated with Cordilleran deformation, resulting in fluid consumption by rock alteration during deformation. Because thick-skinned thrust belts have less internally sourced fluids than thin-skinned thrust belts, they may be more dependent on external fluids to catalyze deformation.
GEOMETRIES AND KINEMATICS OF THE BASEMENT-INVOLVED LARAMIDE OROGENY
The classic Rocky Mountain Laramide orogen of the coterminous U.S. (“Rockies” for short: Fig. 1) formed a foreland salient in the Phanerozoic North American Cordilleran orogen that stretches from Montana to Mexico. Minor faults (Erslev and Koenig, 2009; Yonkee and Weil, 2015; Fig. 1), balancing of fault-related-folds (Erslev, 1993; Neely and Erslev, 2009), and AMS studies (Weil and Yonkee, 2012; Weil et al., 2014) all indicate E–W to NE–SW shortening, suggesting that the Laramide orogen was dominated by ENE–WSW compression (Blackstone, 1940; Lowell, 1983; Stone, 1986, 1993; Oldow et al., 1989; Brown, 1993; Erslev, 1993; Erslev and Koenig, 2009; Chapin et al., 2014; Yonkee and Weil, 2015; Lacombe and Bellahsen, 2016).
Models of Laramide Deformation in the Lower Lithosphere
Until recently, the paucity of geophysical imaging of the lower crust and lithospheric mantle in the Rockies has hindered attempts to connect Laramide upper-crustal shortening with the subduction of the Farallon Plate. As a result, models of Laramide deformation in the lower crust and lithospheric mantle (Fig. 2) have varied greatly, with some invoking major deformation throughout the Rocky Mountain lithosphere and others restricting deformation to the crust.
Lithospheric faulting models for Laramide arches call on higher-angle normal faults (Stearns, 1971; Fig. 2A) or non-listric reverse faults (McQueen and Beaumont, 1989) that cut the entire crust under Laramide arches (Fig. 2A). These models predict large Moho offsets that were not imaged by previous geophysical studies (Smithson et al., 1979; Brewer et al., 1980; Hurich and Smithson, 1982; Bloxsom Lynn et al., 1983; Sharry et al., 1986). They also predict minimal crustal shortening (or extension, in the case of models invoking major normal faults) that is inconsistent with surface data indicating ~50 km of upper-crustal shortening in Wyoming (Brown, 1993; Erslev, 1993; Stone, 1993).
Models invoking localized brittle thickening and shortening in the upper crust overlying equivalent ductile thickening in the lower crust and lithospheric mantle (Egan and Urquhart, 1993; Fig. 2B) predict crustal roots with a downbowed Moho below individual Laramide arches. Seismic images and the isostatic gravity data indicate, however, that Laramide arches are mostly root-less (Hurich and Smithson, 1982; Hall and Chase, 1989). Lithospheric models that call on regionally distributed pure-shear thickening of the entire lower lithosphere (Kulik and Schmidt, 1988; Stone, 1993) are hard to test, because initial lithospheric thicknesses are unknown.
Lithospheric buckling models (Tikoff and Maxson, 2001; Tikoff et al., 2022; Fig. 2D) deform the entire lithosphere into rhythmic buckle folds, which vary in wavelength according to the thickness(s) of the buckling layers. On an arch-to-arch scale, this hypothesis predicts Moho lows under Laramide basins and Moho highs under Laramide arches, for which there have been both supportive (Malahoff and Moberly, 1968) and contradictory (Hurich and Smithson, 1982) gravity interpretations. The lithospheric buckling model suggests that the relatively flat Moho under the Wind River Arch imaged by data of the Consortium for Continental Reflection Profiling (COCORP) (Brewer et al., 1980; Sharry et al., 1986) originated either as a pre-Laramide Moho low that was subsequently flattened by Laramide lithospheric buckling of the opposite deflection or by semi-independent, counterbalancing Laramide fold wavelengths.
In contrast to the whole-lithosphere models described above, deformation in crustal detachment models (e.g., the “orogenic float” hypothesis of Cordilleran orogenesis; Oldow et al., 1989; Fig. 2C) is largely restricted to the crust. For the Wind River Arch, a major Laramide detachment in the lower crust provides a mechanism to transform ~20 km of WSW-directed slip on the Wind River thrust (Smithson et al., 1979; Brewer et al., 1980; Sharry et al., 1986) without cutting or deflecting the Moho. Restorable structural models of the Wind River Arch uniformly invoke a listric master thrust that soles into a lower-crustal detachment (Kanter et al., 1981; Erslev, 1986, 1993; Sharry et al., 1986; Groshong and Porter, 2019).
Regionally, the anastomosing pattern of Laramide arches (Lowell, 1983; Fletcher, 1984; Oldow et al., 1989; Erslev, 1993) can be explained as fault-related folding on master faults that sole into a lower-crustal detachment. A regional Laramide detachment in the lower crust can explain the apparent linkage of deformation between adjacent Laramide arches. For instance, the Beartooth Arch decreases in amplitude southward as the Bighorn Arch increases in amplitude; and farther to the south, the Bighorn Arch decreases in amplitude as the Owl Creek and Black Hills arches increase in amplitude (Fig. 1). A regional crustal detachment above the Moho is supported by the relatively flat Moho interpreted from the EarthScope Transportable Array seismic data (USArray: Lowry and Pérez-Gussinyé, 2011; Gilbert, 2012; Marshak et al., 2017) regardless of the positions of seismic stations in the overlying Laramide arches.
3D Seismic Imaging of Bighorn Arch Geometries by the NSF/EarthScope Bighorn Project
The 2009–2013 NSF/EarthScope Bighorn Project was designed to test Laramide structural hypotheses with detailed geologic investigations and crustal-scale geophysical imaging of the Bighorn Arch and Bighorn Basin of north-central Wyoming, and the Powder River Basin in northeast Wyoming and adjoining Montana (Figs. 3–5).
The Bighorn region is an outstanding laboratory for studying basement-involved foreland deformation due to excellent surface exposure generated by Quaternary regional uplift (Pierce and Morgan, 1992) associated with the Yellowstone magmatic center. In addition, outcrops and petroleum exploration data define the geometry of a widespread blanket of pre-Laramide sedimentary rocks (Blackstone, 1990; Fig. 6A) that reveals Laramide upper-crustal structures. Furthermore, the region is relatively untouched by other Phanerozoic tectonic events, with Pennsylvanian–Permian deformation from the Ancestral Rocky Mountain orogeny limited to interformational thickening and thinning of late Paleozoic sedimentary rocks (Simmons and Scholle, 1990). Furthermore, Neogene extension in the Bighorn region (O’Rourke et al., 2015) appears to be minimal, with no extensional surface offsets listed in U.S. Geological Survey Quaternary fault catalogs (Machette et al., 2001).
The Bighorn Project’s Bighorn Arch Seismic Experiment (BASE) used the 3D geometry of the lower crust to test hypotheses for the formation of basement-cored arches because these hypotheses predict very different lower-crustal geometries (Fig. 2). Details of the geophysical methods used to develop seismic images and other geophysical models of the region are contained in Yeck et al. (2014) and Worthington et al. (2016). Here, we outline the main results from these geophysical analyses before focusing on their implications to Cordilleran tectonics.
Cross sections (Fig. 5) and a 3D map of the Moho (Fig. 6B) were developed from teleseismic P-wave receiver functions calculated from data collected during the passive phase of the BASE seismic experiments (Yeck et al., 2014; Fig. 5A). These reveal a symmetric upwarp in the Moho where the crust thins from as much as 50 km to 37 km or less between the southern and northern arch plunges. This upwarp extends beyond the Bighorn Arch into the relatively less-deformed Powder River Basin to the north–northeast, where the Moho reaches its shallowest depth. It is important to emphasize that this N-trending Moho upwarp, as seen in the central east–west transect across the range, does not parallel the NNW-trending Laramide Bighorn Arch defined by Phanerozoic strata. Instead, the Moho upwarp is characterized by a closely spaced series of N–S trending crustal highs that are discordant to the Laramide arch (Yeck et al., 2014).
During the active portion of the BASE seismic experiments, data from 24 single-fire shots were recorded across the entire receiver array (Fig. 4), from which 2D seismic velocity models of the crust and upper mantle were developed (Fig. 5). Here, we focus on results from the more E–W-oriented transect across the range along which gravity and magnetic models were also developed (Worthington et al., 2016; Fig. 5). These models document the upwarp of the Moho beneath the central Bighorn Arch along Line 1 (Fig. 4) and are consistent with observed teleseismic receiver function results. In addition, these models define the geometries of the Bighorn and Powder River basins, and document the presence of material with velocities greater than 7 km/s in the lower crust beneath both the Bighorn Basin and the eastern Powder River Basin.
Prior to BASE, our knowledge of the crustal velocity structure in the Bighorn region came from the 1995 regional Deep Probe seismic experiment (Henstock et al., 1998; Snelson et al., 1998), which crossed the western Bighorn Basin in a NNW–SSE orientation. The velocity and gravity models from that work revealed a ca. 20-km-thick, high-velocity (> 7 km/s) layer at the base of the crust, which Snelson et al. (1998) referred to as the 7.X layer and interpreted as mafic material underlying more silicic crust.
The integrated interpretation of BASE results (Worthington et al., 2016) along the E–W Line 1 transect (Fig. 5) calls for a more complex lower crust. Whereas the 7.X layer does occur beneath the western Bighorn Basin and eastern Powder River Basin, it is absent or very discontinuous beneath the Bighorn Arch itself. This contrasts with the 7.X layer that was imaged for hundreds of km in a N–S sense by Deep Probe and was presumed to be characteristic of the entire Wyoming craton (Snelson et al., 1998). Further, significant changes in both seismic reflectivity and magnetic susceptibility of the crust on the eastern side of the Bighorn Arch led Worthington et al. (2016; see also Bedrosian and Frost, 2022) to suggest that the eastern Bighorn Arch may contain the margin of a major Proterozoic orogenic suture, whose shear zones may have nucleated the Bighorn Arch during the Laramide orogeny. Co-located changes in the character of shear-wave splitting derived from BASE passive recordings suggest that the proposed Proterozoic boundary extends into the lithospheric mantle (Anderson et al., 2014; Birkey, 2022).
As for testing models of Laramide arch formation, neither the passive nor the active seismic results image offsets of the Moho, which rules out lithospheric models that invoke major crust-cutting faults. The lack of evidence for a crustal root beneath the Bighorn Arch (Fig. 5B) similarly rules out localized lithospheric thickening under the arch (Egan and Urquhart, 1993; Fig. 2B). Neither the Moho geometry nor the absence of horizontal reflectivity in single-fold seismic records support hypotheses that invoke regional ductile thickening of the lower crust by Laramide ductile shear. Furthermore, shear-wave splitting analyses produce heterogeneous results with no consistent alignment relative to Laramide compression directions, which might have been expected during pervasive penetrative shortening of the crust and mantle (Anderson et al., 2014; Birkey, 2022).
BASE single-fold seismic data (Worthington et al., 2016) also lack reflections that could be associated with a lower-crustal detachment or a Bighorn master fault. This absence of reflectivity from these predicted faults is consistent with higher-resolution, multi-fold reflection data from the Rockies, in which reflections from sedimentary strata are far stronger than Laramide fault plane reflections (Stone, 1993). Even the industry-style, multi-fold seismic reflection data from the COCORP Wind River experiment (Smithson et al., 1979) could only image fault plane reflections to a depth of ~20 km. Thus, in our opinion, the absence of lower-crustal reflections does not preclude the presence of a lower-crustal detachment (Fig. 2C).
As we noted, the middle arch-normal transect does show a symmetric, gently upwarped Moho centered on the axis of the Laramide Bighorn Arch (Yeck et al., 2014; Fig. 5); Worthington et al., 2016). While that upwarp appears to be consistent with Laramide lithospheric buckling, its geometry differs from typical anticlinal buckle folds that tighten with depth (Dahlstrom, 1990). The arch in the Moho beneath the Bighorn Mountains is much gentler, not tighter, than the overlying Laramide arch defined by sedimentary strata (Fig. 5A). And in 3D (Fig. 5), it is highly discordant to the NNW–SSE-trending, crescent-shaped Laramide arch.
The east-dipping Moho under the west-dipping Paleozoic strata in the eastern Powder River Basin cannot be explained by a single phase of deformation as the flattening of the Paleozoic strata increases, not decreases, the Moho’s easterly dip. Instead, these observations suggest discrete Precambrian and Phanerozoic episodes of folding (Yeck et al., 2014; Worthington et al., 2016), consistent with the conclusion of Worthington et al. (2016) that the Moho geometry beneath northeastern Wyoming is largely a relic of Precambrian processes.
Recently, Tikoff et al. (2022) used the observation of an arch in the Moho beneath the central Bighorn Mountains to argue for Laramide lithospheric buckling (Tikoff and Maxson, 2001). As they discuss, buckling of cratonic lithosphere would be expected to create arch wavelengths of ca. 400 km (Cloetingh et al., 1999), which is consistent with basin-to-basin distances east of the Rocky Mountains in the central North American craton. To support equivalent Laramide fold wavelengths, Tikoff et al. (2022, their fig. 8) present a cross section that shows a distance of ~240 km between the crests of the Wind River and Bighorn arches. Unfortunately, this cross section omits the Owl Creek Arch, which occurs midway between these arches where it defines the divide between the Wind River and Bighorn basins (see Stone, 1993; Fig. 3). When the Owl Creek Arch is taken into consideration, the distance between arches is more like 120 km, consistent with Laramide-arch wavelengths recorded by Erslev (1993), Stone (1993), Marshak et al. (2017), and Craddock et al. (2022, their fig. 29, which contains four to five arches over its 640 km length). All of the above papers show arch spacings that are consistent with the expected wavelengths for crustal buckling (Erslev, 2005), as are the arch spacings in Laramide analog orogens like the Sierras Pampeanas of Argentina (Jordan and Almendinger, 1986; Ramos et al., 2002).
We agree with Tikoff et al. (2022) that Laramide deformation is consistent with endloading from the west. And considering the evidence for lithospheric buckling east of the Rockies, it is reasonable to hypothesize that the Laramide arches of the Rocky Mountains may have had a component of lithospheric buckling. But we maintain that the distinctively different wavelengths of Laramide arches relative to the basins to their east (Marshak et al., 2017) are more consistent with buckling due to crustal detachment than buckling of the entire lithosphere.
In summary, Bighorn Arch seismic, industry, and outcrop data (Blackstone, 1993; Erslev, 1993; Aydinian, 2020) show that the upper crust deformed independently of the Moho and its underlying mantle lithosphere during Laramide deformation. In our view, the only viable way to explain this observation is to separate the profound Laramide deformation of the upper crust from the dramatically less-deformed Moho with a regional Laramide detachment in the lower crust.
Bighorn Arch Kinematics
To further define the crustal detachment hypothesis, important questions concern: (1) the detachment slip direction; and (2) the depth-to-detachment. Slip indicators (e.g., slickensided faults and magnetic fabrics; Aydinian, 2020), balanced restorations, and serial cross sections that used space as a proxy for time (Watkins et al., 2017) were used to address these questions.
Laramide Slip Indicators
In Wyoming, regional fracture and paleomagnetic studies (Erslev and Koenig, 2009; Yonkee and Weil, 2015) have yielded consistent Laramide ENE–WSW slip and compression directions. Deviations from this trend have been attributed to variable arch trends (Neely and Erslev, 2009), basement anisotropies (Weil et al., 2014), and localized slip partitioning (Aydinian, 2020).
The Bighorn Project measured 1,738 sets of slickensided fault plane and lineation orientations with slip indicators (Petit, 1987) in outcrops of Mesozoic strata that post-date the late Paleozoic Ancestral Rocky Mountain orogeny and pre-date the Laramide orogeny (Aydinian, 2020). Ideal compression directions (σ1: Compton, 1966) were calculated from minor faults that did not parallel bedding or clearly evident pre-existing fractures. A 20° angle between slip and maximum compression direction (α) was used in Compton (1966) σ1 calculations, consistent with conjugate slip relationships in > 20,000 minor faults from the Rockies (Erslev and Koenig, 2009).
Rose diagrams of slickenside strikes from fault planes in the Bighorn Arch, when combined with ideal σ1 trends, show clear thrust and strike-slip conjugate relationships (Fig. 7A). The greater proportion of strike-slip faults on arch plunges (Fig. 7B) suggests distributed strike-slip tearing between the more-shortened arch culmination and the less-shortened arch plunges (Furner, 1990; Stone, 2003). Large ~N80W–S80E fault lineaments in the NNW-trending arch plunge are consistent with left-lateral strike slip between less-deformed crust to the north and more-deformed crust to the south (Aydinian, 2020).
Ideal σ1 directions (Compton, 1966; Fig. 7A) mostly trend ENE–WSW and are generally bracketed by the strikes of strike-slip faults or sub-perpendicular to thrust fault strikes. Variations in ideal σ1 directions are symmetric to the Bighorn Arch, with more E–W-trending directions in the SSE arch plunge and more NE–SW-trending directions in the NNW arch plunge. This can be attributed to gravitational spreading of the arch culmination during late-stage arching, consistent with observations of out-of-the-basin thrusting preceding into-the-basin thrusting in both the eastern (Neely and Erslev, 2009) and western (Stanton and Erslev, 2004) Bighorn Basin. The N80W–S80E fault lineaments in the NNW plunge of the Bighorn Arch probably disproportionately absorbed E–W slip, resulting in more NE–SW-oriented residual slip on adjacent minor faults (Aydinian, 2020).
Balanced Restorations of Laramide Geometries
Because both Laramide crustal detachment (Fig. 2C) and lithospheric buckling (Fig. 2D) models require detachment, with the first calling on detachment within the lower crust and the latter calling on a detachment zone near the base of the lithosphere, determining the depth-to-detachment is a critical test of these hypotheses. Depth-to-detachment during volume-constant detachment equals the excess area (the area between a bedding horizon defining the arch and a line connecting this horizon’s lowest points on either side of the arch) divided by its net slip (Chamberlin, 1910; Epard and Groshong, 1993). For the Bighorn Arch, net slip was estimated by line-length-constant unfolding of arch-perpendicular cross sections using Midland Valley 3D MOVE™ software.
In the Rockies, depth-to-detachment is best estimated in arches like the Bighorn where erosion did not remove all sedimentary strata from arch culminations, and closely adjacent sedimentary outcrops allow reliable projection of the Cambrian unconformity (Blackstone, 1993). Where Phanerozoic cover was eroded in the Bighorn Arch, un-reset apatite fission tracks (Giegengack et al., 1990; Peyton and Carrapa, 2013) indicate that the crystalline basement was not deeply eroded.
The best-constrained upper-crustal cross section for the Bighorn Arch comes from Stone (1993), who meticulously compiled surface, seismic, and well data into an ENE–WSW cross section spanning northern Wyoming. This section crosses the Bighorn Arch (Figs. 3, 8) at its culmination in the Piney Creek salient between Sheridan and Buffalo, Wyoming, where a Gulf exploration well intersected the Bighorn master fault that placed basement on Late Cretaceous strata (Furner, 1990; Stone, 1993, 2003). The excess area (510 km2) and line-length shortening (17 km) from this section gives a calculated depth-to-detachment of 30 km, consistent with: (1) BASE seismic data that requires the detachment to be above the unbroken Moho (Yeck et al., 2014; Worthington et al., 2016); and (2) the 31 km depth-to-detachment calculated for the Wind River Arch (Groshong and Porter, 2019).
Potential errors in this depth-to-detachment calculation include cryptic layer-parallel shortening (LPS) that has been revealed by anisotropy of magnetic susceptibility (AMS) data (Weil and Yonkee, 2012). Ignoring this LPS will cause an underestimation of total slip and a corresponding over-estimation of the depth-to-detachment. Arch-scale lithospheric flexure due to thrust loading, as indicated by the westward dip of strata under the basement overhang of the Piney Creek salient (Stone, 1993), will cause an underestimation of excess area and a corresponding under-estimation of the depth-to-detachment. Regardless of how these errors counterbalance each other, the lack of imaged Moho involvement underneath the Bighorn Arch indicates detachment within the lower crust, instead of near the base of the lithosphere.
Incremental Cross-Section Restorations
Bighorn Project fault data (Aydinian, 2020) and previous depth data compilations for structure contour maps (e.g., Blackstone, 1993; Naus, 2000) tested the hypothesis of a ~30-km-deep Laramide detachment with incremental restorations of ENE–WSW cross sections through the Bighorn Arch (Stone, 1993, his fig. 8; Yeck et al., 2014, their fig. 9, line 1; Worthington et al., 2016). MOVE™ software modeled rotational-fault-bend folding (Erslev, 1986) on a listric master fault with fault-parallel flow in the hanging wall and trishear fault-propagation folding at the fault tip (Erslev, 1991; Zehnder and Allmendinger, 2000). To merge the listric master fault with a sub-horizontal detachment, an asymmetrical trishear zone placed most of the deformation in the hanging wall.
MOVE™ restorations (Figs. 8, 9) used 1 km slip increments on the master fault. For the cross section (Fig. 8) based on Stone (1993), 6 km of fault-parallel flow was used to restore the continuity of the sediment-basement interface, undoing the thrust overhang of the Piney Creek salient. An additional 12 km of fault-parallel flow with trishear fault-propagation folding at the fault tip was then used to unfold the hanging wall while restoring the master fault tip to a horizontal lower-crustal detachment.
The hypothesized sequence of Laramide deformation is the inverse of this restoration, with the listric master thrust initially propagating from a lower-crustal detachment toward the surface with fault-propagation folding at the fault tip. When the thrust tip propagated through the basement unconformity, fault slip and associated fault-parallel-flow folding created the basement overhang at the Piney Creek salient. This model replicates the major features of the Bighorn Arch, including the consistent westward stratal tilts in the eastern and central Bighorn Basin.
The Bighorn Project’s BASE Line 1 (Fig. 9) was restored using a similar combination of fault-parallel-flow and trishear-fault-propagation folding as a listric master thrust propagated from a regional detachment at a depth of ~30 km. On this profile, however, there is no evidence that the master Bighorn thrust cut the sediment-basement unconformity. While thrust faults have been placed along the eastern margin of the Bighorn Arch (e.g., Buffalo deep thrust; Stone, 1993), proprietary seismic profiles show that many of these faults are generated by localized synclinal crowding, not by the breakthrough of the master Bighorn thrust. To honor these geologic constraints, the BASE Line 1 cross-section restoration used a reduced fault-propagation so the tip of the master thrust did not propagate through the basement unconformity and create a basement overhang. As a result, Laramide deformation in the hanging wall of the BASE Line 1 profile was largely accomplished by fold shortening in the arch forelimb.
The rough equivalence between the thrust and fold shortening in the Piney Creek salient (Stone, 1993) and the fold shortening to its north and south suggests that the master thrust propagated farther, and perhaps faster, beneath the Piney Creek salient. This is consistent with the thrust following a pre-existing weakness (Worthington et al., 2016) in the Piney Creek area. Thus, the Bighorn Arch may have nucleated on a pre-existing weakness that could propagate more rapidly to the surface to form the Piney Creek thrust salient and less rapidly on strike, where deformation formed flanking forelimbs dominated by fold shortening.
Pre-existing weaknesses can also explain differences in backlimb deformation between the Stone section (1993; Fig. 8) and BASE Line 1 (Fig. 9) profiles in the eastern Bighorn Basin. The Stone (1993) section is dominated by WSW-dipping thrusts, whereas the BASE Line 1 profile contains both ENE- and WSW-dipping thrusts (Stanton and Erslev, 2004). This change in structural style can be ascribed to different orientations of pre-existing weaknesses in the same zone of backlimb shortening (Erslev, 1986). The existence of reactivated basement weaknesses is confirmed by thinned and absent late Paleozoic reservoir units in anticlinal crests within the backlimb (Simmons and Scholle, 1990).
Restoration by Trading Space for Time
Parallel regional cross sections with progressively increasing amounts of shortening were used to infer the evolution of Laramide arch formation by using the space-as-aproxy-for-time approach (Watkins et al., 2017). This technique proposes that less-deformed sections represent earlier deformation, whereas more-deformed sections represent the geometric summations of both earlier and later deformation. This technique is ideally suited for the northern half of the Bighorn Arch due to the continuous increase in arch amplitude (and thus shortening) southward from a broad crustal warp near Billings, Montana, to the arch’s maximum amplitude near the Piney Creek salient between Sheridan and Buffalo, Wyoming.
Serial cross sections through the Bighorn Arch were constructed using MOVE™, with in-plane motion maximized by constructing sections parallel to N60E–S60W average slip and compression directions indicated by fault slip data (Fig. 7A). Figure 10 shows the array of sections cutting the 3D MOVE™ model and four representative sections that combined observed fold geometries with outcrop and structure contour data for the basement-cover unconformity, the Pennsylvanian Tensleep Formation (Fig. 6A), and Late Cretaceous Frontier Formation. These serial sections (Fig. 10) show the transition from a symmetric, low-amplitude arch into a more asymmetric, higher-amplitude arch. Laterally discontinuous, more steeply dipping Mesozoic stratal panels on both arch flanks can be traced into thrust faults cutting Paleozoic rocks. These panels appear to be the forelimbs of secondary fault-propagation folds that contributed to arch shortening, even though they did not always conform to the arch’s ENE-vergent geometry. Their thrust faults probably nucleated on laterally restricted, pre-existing weaknesses that were in permissible orientations for Laramide thrusting (Ferrill et al., 2016).
Depths-to-detachment for these serial cross sections, as calculated from original bed lengths determined by stratal unfolding combined with the simplest depth-to-detachment equations (Chamberlin, 1910), decrease from ~1,000 km beneath the least-shortened sections south of Billings to ~30 km near the arch culmination. The lack of evidence for Moho-cutting faults shows that additional factor(s) impacted depth-to-detachment calculations (Groshong and Epard, 1994). Adding an initial stage of 2–5% cryptic LPS (as suggested by AMS data of Weil and Yonkee, 2012) brings all calculated depths-to-detachment to lower-crustal levels. This unaccounted LPS helps explain why incremental restorations of regional cross sections (Figs. 8, 9) left bulges of excess material above the basement unconformity. These bulges probably reflect a combination of LPS and deep-seated duplexing, which are both characteristics of early stage detachment folding (Dahlstrom, 1990).
Like all balanced sections, our restorations are non-unique, because a multitude of variables like the master fault’s exact trajectory through the crust, variations in heterogeneous trishear angles, and incremental fault-tip propagation rates make unique solutions impossible. But these restorations bear a strong resemblance to sequential models of fault-related folding in thrust belts derived from direct observations (Dahlstrom, 1970; Elliott, 1976; Price et al., 1981), as well as from numerical (Gray et al., 2014) and analog models (Dixon and Liu, 1992; Poblet and McClay, 1996).
Our proposed model of Bighorn Arch formation by crustal detachment (Fig. 11) uses an overlapping sequence of detachment folding + LPS (Stage 1) and fault-propagation and fault-bend folding (Stage 2). When combined with geophysical evidence for a Proterozoic mobile belt along the eastern margin of the Bighorn Arch (Worthington et al., 2016), this model suggests that the Bighorn master fault nucleated on a pre-existing Precambrian weakness that tapped into a regional, ENE-slipping detachment in the lower crust.
The absence of fast 7.X km/sec lower crust under the central Bighorn Arch (Worthington et al., 2016) may have been another factor determining detachment nucleation. Lower velocity (< 7 km/s) and thus probably weaker lower crust may have concentrated crustal thickening below the Bighorn Arch, similar to the nucleation of large Appalachian anticlines in areas with thick ductile shale (Thomas, 2001). Additional seismic studies could test this hypothesis by determining whether lower-velocity crust consistently underlies other foreland arches.
Regional Kinematics of Laramide Crustal Detachment
The Bighorn Arch (Fig. 6A) and its slip system (Fig. 7B) resemble the “bow and arrow” geometries of thin-skinned thrust-belt anticlines (Elliott, 1976), where maximum horizontal slip occurs in the middle of anticlinal crests that are bowed in the direction of slip. This geometry, and the similar geometry in the nearby Black Hills Arch (Lisenbee and DeWitt, 1993; Singleton et al., 2019), support the hypothesis of regional ENE-directed Laramide crustal detachment.
With structural restorations and seismic tomography of the Bighorn Arch indicating detachment between 25 and 35 km depth beneath the Bighorn Arch and COCORP data indicating thrust splays extending to 28 to 30 km depth beneath the Wind River Arch (Lynn et al., 1983; Sharry et al., 1986), the hypothesized regional Laramide detachment must be, on average, essentially flat. Maximum and minimum estimates of depth-to-detachment under the Wind River and Bighorn arches suggest that the current dip of the regional detachment is less than one degree. A near-horizontal detachment is consistent with the fact that Laramide arches are typically bivergent to either the ENE or WSW, consistent with them merging into a master detachment. The generally west-to-east initiation of Laramide arches in both the Southern Rocky Mountains (Caylor et al., 2021; Thacker et al., 2022; Davis et al., in press) and Northern Rocky Mountains (Perry et al., 1992; Flores, 2003; Carrapa et al., 2019; Garber et al., 2020; Orme, 2020) indicates that the detachment slipped and propagated ENE. Crustal detachment probably started to the west beneath the Sevier thrust belt, whose earliest thrusting preceded Laramide foreland deformation (Hunter, 1988; DeCelles, 2004). This suggests that the Laramide detachment in the Northern Rockies is rooted in the mantle under the main Cordilleran thrust belt, where it may have merged with Sevier thrusting and probably has been concealed by Neogene normal faulting. If the detachment had been rooted under the mid-continent in a west-vergent system, slip of this magnitude should have been revealed by Earth-Scope USArray seismic data (Porritt et al., 2014).
This detachment hypothesis suggests that while the extent of flat slab subduction (Liu et al., 2010) defines the larger location of the Laramide orogeny, the abrupt edges of Laramide deformation (Marshak et al., 2017) primarily follow the margins of an allochthonous sheet of upper crust, not any regional changes in crustal rheology (Marshak et al., 2000; Timmons et al., 2001; Chapin et al., 2014).
Origin of Upper-Crustal Constriction in the Rockies
Gries (1983) used industry data to document crustal shortening across Laramide arches with a wide variety of trends. While fault and fold trends have been used to support multi-stage, multi-directional Laramide compression (Chapin and Cather, 1981; Bergh and Snoke, 1992; Wise, 2000), regional evidence for a consistent sequence of compression directions is lacking (Erslev and Koenig, 2009; Yonkee and Weil, 2015). In fact, the interconnected geometry of the anastomosing Laramide arches suggests locally synchronous, not sequential, Laramide slip (Erslev, 1993; Molzer and Erslev, 1995).
Bump (2004) invoked regional constriction, where shortening occurred in all horizontal directions, to explain the shortening across a similarly diverse array of Colorado Plateau monoclines. He hypothesized that constriction resulted from the combination of horizontal shortening from low-angle subduction with oblique shortening from the spreading of topographic highs in the Sevier thrust belt, which wraps around the western Colorado Plateau.
Borrowing this concept from Bump (2004), we propose that the anastomosing web of Laramide arches (Fig. 1) in the Northern Rockies also resulted from constriction. In this case, constriction may have resulted from the wedge-like shape (in map view) of the detached Laramide allochthon. The northeastern margin of the proposed Laramide allochthon is defined by the northern plunges of the en échelon Beartooth, Bighorn, and Black Hills arches, which also parallel en échelon lineament arrays like the Nye-Bowler, Lake Basin, and Cat Creek lineaments in Montana that indicate sinistral Laramide motion (Thom, 1923; Wilson, 1936; Chamberlin, 1945; Foose et al., 1961; Johnson et al., 2005). This zone of shear follows the Proterozoic Perry fault on the southern margin of the Belt Basin (Harrison et al., 1974; Winston, 1986), suggesting that the northeastern boundary of the Laramide allochthon was at least partially defined by reactivated Precambrian weaknesses.
The Laramide orogen’s southeastern boundary is parallel to the similarly en-échelon southern plunges of the Sangre de Cristo, Wet Mountain, Front Range, and Laramie arches (Fig. 1). This Laramide boundary locally parallels large Proterozoic strike-slip faults (Karlstrom and Daniel, 1993; Fankhauser and Erslev, 2004) as well as a proposed N–S Mesoproterozoic structure (Chapin et al., 2014). The en échelon steps between these arches suggest a component of dextral Laramide motion, as do paleomagnetic rotations in NE-striking Laramide fault zones (Tetreault et al., 2008) that locally extend into Precambrian shear zones (Erslev and Larson, 2006).
Thus, substantial sections of the Laramide orogenic margin may follow pre-existing Precambrian structures. Connecting these boundaries to a detachment at ~30 km depth defines a wedge-shaped allochthon that narrows to the ENE. Detaching this sheet of crust in the ENE slip direction should cause constriction by combining 1) regional ENE–WSW tectonic shortening with 2) NNW–SSE shortening due to the progressive narrowing of the wedge margins to the ENE. The component of NNW–SSE shortening within the wedge appears to be preferentially released in southwest and central Wyoming by oblique slip in the more east-striking Uinta, Sweetwater, and Owl Creek arches, all of which parallel Precambrian fabrics (Brown, 1993).
This wedging of the northern Laramide foreland can be visualized as the inverse of splitting wood with a steel wedge. A steel wedge splits wood because it is much stronger than the wood. In contrast, the detached Laramide crustal allochthon is thin and much weaker than the surrounding autochthonous cratonic lithosphere. As a result, this weak, wedge-like allochthon (Fig. 12) can be expected to deform internally, which could generate the anastomosing array of constrictional Laramide arches.
Globally, analogous basement-involved foreland orogens can be expected to be bounded by shear zones between allochthonous and autochthonous continental crust. These shear zones can be expected to converge toward the craton, because oblique shear zones have greater shear stresses than compression-parallel shear zones. The amount of constriction may depend on the angles between tectonic shortening and pre-existing weaknesses. For instance, the smaller proportion of anastomosing arches in the Sierras Pampeanas orogen may be due to the orthogonality of shortening directions to the strikes of pre-existing Paleozoic sutures (Pearson et al., 2013).
IMPLICATIONS OF LARAMIDE CRUSTAL DETACHMENT ON CORDILLERAN FORELAND DYNAMICS
Crustal detachment in Cordilleran forelands challenges some widely held tectonic hypotheses for (1) what drives Laramide-style foreland orogenies; and (2) why Laramide-style shortening of previously stable continental crust occurs preferentially in Cordilleran orogens undergoing low-angle subduction.
Tectonic Driving Forces
There is little consensus as to how basement-involved foreland orogens overcome the hallmark stability of Precambrian cratons (e.g., U.S. Rockies) and other areas of previously stable continental crust (e.g., Sierras Pampeanas). For the Rockies, Dickinson and Snyder (1978) hypothesized that Laramide deformation was caused by traction between the subducting Farallon Plate and the overriding North American lithosphere. Similar hypotheses, many of which invoke increased shear coupling during the subduction of buoyant oceanic plateaus (Livaccari et al., 1981; Henderson et al., 1984; Humphreys et al., 2003; Saleeby, 2003; Liu et al., 2010; Yonkee and Weil, 2015; Lacombe and Bellahsen, 2016; Copeland et al., 2017; Pfiffner, 2017), are appealing because they require relatively short stress and slip transmission distances between the upper crust and a low-angle slab in the mantle.
But numeric models of subduction-driven traction (Bird, 1984, 1988) on the base of the North American lithosphere predict a thickened keel of lower crust east of the Rockies that has not been detected. Moreover, isotopic signatures of Rocky Mountain mantle xenoliths match those of Precambrian surface exposures (Livaccari and Perry, 1993; Jones et al., 2011, 2015), suggesting mostly autochthonous Rocky Mountain lithosphere.
Inverse plate models charting the subduction of oceanic plateaus (Saleeby, 2003; Liu et al., 2010; Humphreys et al., 2015) are problematic for traction models because they predict the transit of oceanic plateaus from beneath the Rockies before peak Laramide deformation (Fan and Carrapa, 2014; Heller and Liu, 2016). Laramide crustal detachment models—as proposed here—worsen this timing mismatch, because they place Laramide driving stresses west of the Rockies at times when subduction models (Liu et al., 2010; Humphreys et al., 2015) place the conjugate Shatsky Plateau east of the Rockies. As a result, Heller et al. (2013) linked the passage of the conjugate Shatsky Plateau to early Laramide conglomerates that record ~1 km crustal undulations, not to later, more major Laramide shortening. Heller and Liu (2016) suggested that plateau subduction may have initiated the plate coupling that would drive Laramide shortening, but no such coupling mechanism has been fully proposed.
Additional challenges for Laramide traction hypotheses come from the discordance between relative plate motions and the fanning pattern of Laramide shortening (Fig. 1). Liu et al. (2010) and Humphreys et al. (2015) determined an ~N25E trajectory for the conjugate Shatsky Plateau while it was below the Rockies. In contrast, Erslev and Koenig (2009) calculated a vector mean of N66E–S66W from Laramide arch, fold, and minor fault orientations in the Middle Rocky Mountains, in agreement with recent structural and thermochronology studies from the southern Colorado Plateau (Davis et al., in press). For traction to be viable, the ~40o of angular discordance between subduction trajectories and crustal shortening trends appears to require some sort of slip partitioning within the continental lithosphere directly above the subducting slab. But Laramide slip partitioning within the Rockies appears to be limited to lateral stress deflections near strike-slip faults (O’Meara, 1996; Erslev et al., 2004; Fankhauser and Erslev, 2004; Erslev and Larson, 2006; Chapin et al., 2014; Weil et al., 2014; Caine et al., 2017; Aydinian, 2020). In addition, traction directions from a subducting slab are unlikely to parallel the fanning Laramide shortening directions that range from nearly E–W in New Mexico to N60E–S60W in southern Montana (Erslev and Koenig, 2009; Fig. 1).
In comparison, end-loading models of Laramide deformation (Oldow et al., 1989; Livaccari, 1991) are consistent with oblique Cordilleran convergence, which is commonly partitioned into trench-parallel strike slip in the hinterland and more trench-perpendicular thrust slip in the foreland (Tikoff and Teyssier, 1994; Fossen and Tikoff, 1998; Craddock and Malone, 2022). Additional stress from trench-parallel topographic highs can explain the more E–W shortening in Sevier thrust belt (Yonkee and Weil, 2015) relative to the more ENE–WSW shortening in the more distal Laramide foreland (Erslev and Koenig, 2009; Davis et al., in press).
In the analogous (and active) Sierras Pampeanas orogen, end-loading crustal detachment hypotheses are supported by relatively planar Moho geometries, crustal microseismicity, and focal mechanisms. Most microseismicity within the crust occurs along a subhorizontal plane at ~20 km depth, which researchers interpret as a mid-crustal detachment (Richardson et al., 2013; Linkimer et al., 2020). And basement arches extend east of where the Andean slab turns steeply downward into the mantle, so the eastern-most arches (e.g., Sierra de Córdoba; Perarnau et al., 2012; Richardson et al., 2011, 2013) must be driven by stresses from the west, not by traction from below. Moreover, focal mechanisms within the subducting Nazca slab show that thrust faulting is only dominant adjacent to the trench down to a depth of ~70 km (Araujo and Suárez, 1994; Pardo and Suárez, 1995). Normal faulting, probably due to slab bending (Anderson et al., 2007), predominates in the slab beneath the Sierras Pampeanas.
Thus, we conclude that thin- and thick-skinned Cordilleran thrust belts in the Rocky Mountains and analogous forelands (Horton et al., 2022) were driven by subduction-related end-loading. Additional stresses from Cordilleran topography (Livaccari, 1991; Bump, 2004; Neely and Erslev, 2009; Smaltz and Erslev, 2013), as well as the reactivation of pre-existing weaknesses and the wedging in foreland allochthons probably added local variability to foreland deformation.
Origin of Continental Weakness in Cordillera Undergoing Low-Angle Subduction
If basement-involved deformation in Laramide and analogous orogens were primarily driven by end-loading, then how were stress and slip transmitted through these cordillera and then so far into their adjoining, previously stable foreland crust? And why does this occur preferentially in Cordilleran orogenies undergoing low-angle subduction? Interpretations of active basement-involved slip in the Franklin Mountains of the northern Canadian Rocky Mountain foreland propose that ductile deformation at elevated crustal temperatures (> 600oC) on a very-low-angle detachment can transmit slip through a Cordilleran backarc (Mazzotti and Hyndman, 2002). Hyndman et al. (2005) concluded that the Cretaceous–Paleogene backarc containing the Canadian thrust belt was ~10 times weaker than the adjacent craton due to subduction-related heating and hydration in the backarc. Hyndman et al. hypothesized (see Fig. 13A and 13A caption) that the Canadian craton was shielded from subduction-related fluids by asthenospheric corner flow and arc volcanism that directed volatiles and heat away from the craton.
Mesozoic arc processes in the Canadian hinterland probably made it hotter and weaker than in the Sevier hinterland, whose arc system was shut down by low-angle subduction. As a result, the Sevier hinterland may have formed a mechanically stronger backstop that was better able to transmit stress and slip from the west to the Rocky Mountain craton. But why did slip propagate so much farther into the previously stable Rocky Mountain craton than it did in the Canadian craton to the north?
More extensive deformation of the Rocky Mountain craton could be explained if it was hotter than the Canadian craton. But fission-track analyses from the Bighorn (Crowley et al., 2002; Peyton and Carrapa, 2013) and other Rocky Mountain arches (Peyton et al., 2012; Stevens et al., 2016) indicate low geothermal gradients of ~20oC/km. Active Laramide analogs in the Andean foreland adjacent to low-angle subduction also have craton-like geothermal gradients (Gutscher and Peacock, 2003; Wagner et al., 2006; Manea et al., 2017) and mantle S-wave velocities consistent with low temperatures (Porter et al., 2012; Linkimer et al., 2020). The predicted 400–500°C lower-crustal temperatures (English et al., 2003; Currie and Beaumont, 2011) in these forelands are probably too low for fully ductile, nearly friction-less detachment.
Moreover, xenoliths (Dumitru et al., 1991; Jones et al., 2015) and geodynamic modeling (Bird, 1988; Currie and Beaumont, 2011) indicate lithospheric refrigeration, not heating, during Farallon low-angle subduction. Because lithospheric cooling should strengthen, not weaken, continental crust (Hyndman, 2017), thermal processes alone are unlikely to resolve the mechanical paradox of basement-involved, Laramide-style foreland deformation.
A distinguishing feature of the Laramide orogeny is the very-low critical-taper angle between surface slopes and the nearly horizontal Laramide detachment plane suggested by our cross-section restorations. A slight eastward dip on the earth’s surface is consistent with synorogenic Paleocene river drainages (Flores, 2003), but closed drainages containing large synorogenic lakes do not suggest substantial regional slopes during the Laramide orogeny. If Laramide arches were driven by a nearly horizontal lower-crustal detachment, then the critical taper of the allochthonous Rocky Mountain upper crust must have been less than a few degrees during Laramide detachment.
Thrusts belts are typically modeled as critically tapered Coulomb wedges whose horizontal driving forces equal their resistance to sliding (Davis et al., 1983). Increasing wedge strength can reduce the critical taper, but pervasive fracturing and folding within the Laramide allochthon should render it weaker than the surrounding autochthonous cratonic crust. This is consistent with the greater flexural rigidity of the Canadian craton west of the Cordilleran thrust belt relative to the flexural rigidity in the U.S. Rocky Mountains (Saylor et al., 2020). A weakened crustal allochthon in the U.S. Rockies would be expected to build a higher-angle critical-taper wedge.
Elsewhere, very-low-angle critical-taper wedges occur when localized in high-ductility salt and shale provinces like the Niger delta (~3o critical taper; Suppe, 2007). But outside of localized Laramide igneous provinces, there is no evidence for widespread high-ductility crust in the Laramide orogen (Currie and Beaumont, 2011; Lowry and Pérez-Gussinyé, 2011). In thrust belts with consolidated, non-ductile strata, one of the lowest critical tapers is in the Taiwan thrust belt (~8o critical taper; Yue and Suppe, 2014).
Other ways to explain thrust belts with single-digit critical tapers is to invoke very weak fault rocks and/or nearly lithostatic fluid pressures (Suppe, 2007). Phyllosilicate-rich detachment zones, most notably talc-bearing zones, can reduce coefficients of friction to levels around 0.01 (Moore and Lockner, 2013). High fluid pressures in detachment zones can also greatly reduce fault friction (Davis et al., 1983), but if elevated fluid pressures are not confined to the detachment zone, they will also increase the critical taper by weakening the thrust wedge.
The possibility that Laramide detachment occurred on an extremely weak, hydrated fault zone is supported by recent Rocky Mountain seismic velocity, xenolith, magmatic composition, and flexural observations indicating large-scale hydration of the lower crust (Humphreys et al., 2003; Jones et al., 2015; Tesauro et al., 2015; Porter et al., 2017; Schutt et al., 2018; Farmer et al., 2020; Saylor et al., 2020). During high-angle subduction, slab fluids catalyze arc volcanism and asthenospheric viscosity reduction (Hyndman, 2017; Fig. 13A), whereas during low-angle subduction, these fluid-consuming processes are shut down (Fig. 13B). The resulting excess fluids from slab dehydration could be expected to move upward into previously stable foreland lithosphere. Indeed, P-wave, S-wave, and Vp/Vs tomographies imaging the slab and overlying lithosphere in South America support cold, hydrated mantle and lower crust underneath the Sierras Pampeanas (Porter et al., 2012; Linkimer et al., 2020; Horton et al., 2022).
Crustal weakening due to the lower-crustal hydration can explain the co-location of more extensive basement-involved foreland shortening with magmatic gaps caused by low-angle subduction. Hydration of the overriding plate during low-angle subduction could also increase fluid pressures by reducing permeability due to higher-temperature minerals converting to lower-density, higher-volume hydrated minerals. In addition, constrictional shortening above a lower-crustal detachment (Fig. 13B) may help trap fluids in the lower crust by closing vertical fractures that could allow fluid escape. In a New Zealand accretionary prism, Arnulf et al. (2021) showed that pore pressure appears to increase where through-going faults do not connect the underlying detachment zone with the ocean bottom.
Slow Earthquakes: A Possible Indicator of Regional Foreland Detachments
A full understanding of the delamination and detachment of large expanses of continental crust requires identifying deformation mechanisms capable of allowing very-weak detachment in lower-crustal conditions at 400–600oC. Recent research on slow earthquake (SE) phenomena like slow-slip events, low frequency earthquakes, and tectonic tremor (Kato, 2011; Obara, 2020) show that they can be associated with shallowly dipping thrust motions (Ide et al., 2007) in hydrated regions of accretionary prisms (Arnuff et al., 2021; Luo and Liu, 2021) and in the lower crust of subduction forearcs.
Slow earthquake (SE) activity was first recognized in the subduction channels of Andean forearcs (Dragert et al., 2001) above areas of slab dehydration, as indicated by anomalously low Vs/Vp seismic velocity ratios. SE activity commonly extends from the landward-edge of the locked earthquake zone to where slip transitions into continuous creep, presumably by higher-temperature ductile processes. Using SE reactions to earth tides, Houston (2015) calculated such low coefficients of friction (< 0.1) that she described SE deformation as “Teflon tectonics.”
Above the central Mexican flat slab, elevated levels of SE activity occur where the top of the slab coincides with the Moho (Manea and Gurnis, 2007). An ~4 km thick, ultra-slow-velocity layer on the flat-slab interface has been attributed (Song et al., 2009; Kim et al., 2012) to hydrated minerals like serpentine and talc. Indications of extreme weakness include SE triggering by distant earthquakes and the paucity of contractional deformation in the overriding plate (Zigone et al., 2012; Manea et al., 2017).
SE phenomena also occur below the San Andreas fault’s upper-crustal seismogenic zone at depths of 16–20 km and temperatures of 450–520°C (Fagereng and Diener, 2011). Abundant fluids are indicated by low Vp, low Vs/Vp, and areas of high seismic reflectivity. Tremor activity is impacted by teleseismic waves and earth tides (Peng et al., 2009; Thomas et al., 2009), again suggesting extremely weak faults.
The regional Laramide detachment proposed here would have occurred at pressures and temperatures (Lee, 2005; Currie and Beaumont, 2011; Jones et al., 2015) similar to those in areas currently experiencing lower-crustal SE deformation. The Sierras Pampeanas have been interpreted as low-angle (< 2°) critical-taper thrust wedges (Alvarado and Ramos, 2011; Ramos et al., 2014), with Vp/Vs studies showing evidence for eastward dehydration of the underlying slab and hydration of the overlying South American lithosphere (Gans et al., 2011; Porter et al., 2012; Linkimer et al., 2020). The abundance of microseismicity along an interpreted mid-crustal detachment at 20-km depth (Richardson et al., 2013) could signify elevated pore pressures.
We propose that SE processes allow end-loading to drive lower-crustal detachment of hydration-weakened continental crust in Cordilleran forelands undergoing low-angle subduction. This hydration-focused alternative to traction-driven foreland shortening can explain the geographic limits to Laramide-style deformation in Cordilleran orogens, while better explaining the pause between plateau subduction and upper-crustal deformation as well as the discordant directions of plate convergence and upper-crustal shortening.
The NSF/EarthScope-funded Bighorn Project confirmed the thrust-dominated, contractional nature of the basement-involved Rocky Mountain Laramide orogeny. Regional structural and seismic data show that ENE-directed crustal detachment decoupled upper-crustal faulting and folding from the minimally deformed Moho. Constrictional shortening within the wedge-shaped (in map view) Laramide allochthon can explain the anastomosing geometry of Laramide arches and their intervening basins.
Previously, Cordilleran deformation of stable continental crust behind magmatic gaps in Andean-style arcs (Lowell, 1974; Barazangi and Isacks, 1976; Dickinson and Snyder, 1978; Jordan et al., 1983) was commonly explained by traction between buoyant subducted slabs and their overlying continental lithosphere. But contradictory plate and foreland convergence directions, discordant deformation timings, and crustal detachment geometries indicate the primary importance of end-loaded stresses for the Laramide orogeny. Other examples of Cordilleran low-angle detachment (Mazzotti and Hyndman, 2002; Wells and Hoisch, 2008) show that stress and slip can be transmitted from near-trench subduction channels through Cordilleran backarcs by higher-temperature ductile slip. Low-angle subduction, however, appears to cool, not heat, adjoining foreland crust, so crustal detachments in Laramide and analogous orogens require a lower-temperature deformation mechanism.
ENE-directed detachment of a thin sheet of Laramide upper crust with a near-zero critical taper appears to necessitate a nearly frictionless detachment. Slow earthquake (SE) phenomena in Cordilleran forearcs (Dragert et al., 2001; Lowry et al., 2001) show analogous very-low-frictional slip under similar highly hydrated lower-crustal conditions.
The association of crustal hydration with SE phenomena can explain the linkage of basement-involved foreland deformation with low-angle subduction. During higher-angle Cordilleran subduction, cratons are shielded from subduction fluids by arc melting and asthenospheric corner flow processes. However, during low-angle subduction, fluids—liberated during subduction—can infiltrate upward into overriding lithosphere (Humphreys et al., 2003). These fluids can weaken overlying crust by catalyzing retrograde metamorphism and reducing effective stresses. We propose that extensive basement-involved foreland deformation occurs during low-angle subduction due to subduction fluids invading and ungluing previously stable continental crust (Fig. 13B).
Slow earthquake (SE) seismicity and crustal movements have preceded large seismogenic earthquakes (Segall and Bradley, 2012; Graham et al., 2014; Obara and Kato, 2016; Uchida et al., 2016; Voss et al., 2018). If basement-involved foreland deformation is associated with SE slip on a lower-crustal detachment, then this slip may be discernable by seismological (Kato and Nakagawa, 2014; Cesca et al, 2016) and Global Positioning System (GPS) instruments during and/or preceding major earthquakes in active Laramide analog orogens. Current GPS data suggest that the highly analogous Sierras Pampeanas orogen is locked (Kendrick et al., 2006; Manea et al., 2017), but if slow-slip detachment events precede the next major Sierras Pampeanas earthquakes, these slow-slip events may forecast these earthquakes.
The Laramide orogen has long provided evidence for the importance of subduction angle in Cordilleran orogens. The hypothesis that basement-involved foreland orogens are controlled by the addition of fluids to continental lithosphere during low-angle subduction provides a new explanation for why Laramide-style basement-involved detachment can extend so far into the center of continents.
This manuscript was greatly improved by reviews by Gary Gray, Michael Wells, and two anonymous reviewers as well as by the insightful comments of Rocky Mountain Geology Science Co-Editors Art Snoke and Ron Frost. This paper was greatly improved by the keen eyes of Rocky Mountain Geology Managing Editor Brendon B. Orr and Copy Editor Robert Waggener. We are indebted to Don Blackstone’s and Don Stone’s pioneering integrations of Rocky Mountain surface and subsurface data. Jim Lowell, Bill Dickinson, and John Oldow provided insightful tectonic templates for the North American Cordillera that have been enhanced by Roy Hyndman, Jason Saleeby, Gene Humphreys, Craig Jones, and Lijun Liu.
Prior studies by Don Wise, Thomas Neely, Nicole Koenig, Laura Kennedy, Heather Stanton, Sara Smaltz, Peter Hennings, Adolph Yonkee, Arlo Weil, Kevin Chamberlain, and Gary Gray provided the basis for the Bighorn Project. Bighorn Project colleagues contributed active seismic (Lindsay Worthington and Kate Miller), passive seismic (Anne Sheehan, Will Yeck, and Megan Anderson), and additional structural (Christine Siddoway and Karen Aydinian) observations and analyses. This work was supported by the EarthScope Bighorn Project in National Science Foundation grants EAR-0843835, EAR-1157150, EAR-0843657, EAR-0843889, and EAR-0844202.